1. Field of the Invention
The present invention relates to an aluminum copper clad material in which an aluminum layer and a copper layer are diffusion-bonded.
2. Description of the Related Art
In lithium ion battery packs for use in hybrid vehicles and various electronic devices, lithium ion batteries are connected in series as necessary, and groups of batteries connected in series are further connected in parallel. Accordingly, a large number of wiring materials for connecting the groups of batteries are used. In lithium ion batteries, the positive electrode terminal is made of an aluminum material and the negative electrode terminal is made of a copper material. Therefore, for a wiring material used for series connection, an aluminum copper clad material in which an aluminum material and a copper material are bonded is suitably used as a raw material thereof. That is, the aluminum layer of a wiring material (lead wire) made from the clad material is connected to the positive electrode terminal of one battery, and the copper layer thereof is connected to the negative electrode terminal of another battery. Such connection makes it possible to prevent electrical corrosion in electrode terminal areas, and makes it possible to easily bond the electrode terminal and the wiring material by resistance welding, ultrasonic welding, or the like.
Also, the aluminum copper clad material is suitably used as, in addition to a wiring material for battery connection, a raw material of a wiring material for a capacitor, an intermediate part for connection between a copper part that has a semiconductor element mounted thereon and an aluminum part such as a radiator and the like.
The aluminum copper clad material is usually produced by stacking an aluminum plate that serves as a source of an aluminum layer and a copper plate that serves as a source of a copper layer, pressure-welding the stacked materials through rolls, and diffusion-annealing the resulting pressure-welded material. For the aluminum plate and the copper plate, annealed materials are usually used for enhanced pressure-weldability. During the diffusion annealing, at the interface between the aluminum layer and the copper layer, a brittle intermetallic compound layer made of an Al—Cu intermetallic compound is inevitably generated. When the annealing temperature is 350° C. or greater, generation and growth of the intermetallic compound layer is significant, thus making it difficult to control the thickness of the intermetallic compound layer through adjustment of a retention time, and the bonding strength between the aluminum layer and the copper layer deteriorates. Therefore, conventionally, when an aluminum layer and a copper layer are directly bonded, diffusion annealing is carried out under annealing conditions of a low temperature of 300° C. or less and a retention time of a few hours.
However, as described above, an aluminum copper clad material in which an aluminum layer and a copper layer are diffusion-bonded by low-temperature diffusion annealing is problematic in that sufficient bonding strength is not obtained. Accordingly, JP S62-46278B and JP H11-156995A propose an aluminum copper clad material for which diffusion annealing can be carried out at high temperatures and that has good bonding strength between the aluminum layer and the copper layer. This clad plate has an aluminum layer and a copper layer that are integrally pressure-welded via a nickel layer and then diffusion-annealed. Copper and nickel as well as nickel and aluminum are unlikely to generate an intermetallic compound during diffusion annealing, and thus a pressure-welded material in which an aluminum layer, a nickel layer, and a copper layer are laminated can be diffusion-annealed at high temperatures, and also the bonding strength of the diffusion-annealed clad material is improved.
Regarding the aforementioned aluminum copper clad material provided with a nickel layer, good bonding strength between the aluminum layer and the copper layer is obtained when the retention time is adequate even if the diffusion-annealing temperature is as high as about 500° C. However, the aluminum copper clad material is problematic in that it is necessary to provide a nickel layer that was originally not necessary, and nickel has poorer electrical conductivity and thermal conductivity than aluminum and copper and is more costly.